Digital radio telephones are used throughout the world and are employed in cellular systems such as the European GSM system and the U.S. IS54 system and the Japanese PDC system. Digital cellular telephones intermittently transmit digital signals with a transmitter and receive digital signals with a receiver. The transmitter operates for a short time slot in a repetitive time frame to transmit speech or other data to a base station and alternates with the receiver operating in a different short time slot to receive speech or other data from the base station. Digital cellular telephones are typically battery powered to facilitate mobility.
One problem with digital cellular telephones is that the transmitter consumes higher power from the battery than the receiver. This causes the current drawn from the battery to be cyclic at the frame repetition rate. This cyclic current gives rise to time-varying magnetic field that may cause magnetic coupling with nearby devices. This magnetic coupling may degrade or otherwise interfere with the optimal operation of devices sensitive to time-varying magnetic fields.
One example of a device that is sensitive to time-varying magnetic fields is a hearing aid equipped with a "tele-coil." A "tele-coil" is an inductive device that is placed near a telephone speaker and used to couple audio signals from the speaker to a hearing aid. In simple terms, the tele-coil includes a pick-up coil that is placed into the magnetic field of the speaker. As the speaker is induced to create audio signals by electrical signals sent to its electromagnet, the corresponding magnetic field is coupled to the pick-up coil. This magnetic field is amplified and sent to the hearing aid's speaker or transducer. Thus, a tele-coil facilitates the use of telephones by people who have hearing problems.
Unfortunately, the magnetic coupling that occurs between a digital cellular telephone and the tele-coils of certain hearing aids may generate noise or hum in the hearing aid, making use of the telephone less than optimal for hearing aid users. The noise or hum is at least partially caused by the time-varying magnetic field generated by the varying current levels in the battery-transmitter electrical circuit. This problem of hum is particularly exacerbated when the on/off cycle time of the transmitter is in the audio frequency range.
Thus, there is a need for reducing the effect that magnetic field emissions from a radio telephone may have on devices sensitive to time-varying magnetic fields. One approach is to reduce the strength of the generated magnetic fields by significantly reducing the current flowing through the telephone. However, this has proved impractical based on the power levels required for adequate transmission of signals.